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- From 35b6173e6176fc978c635f9e07f1778eff7b76e7 Mon Sep 17 00:00:00 2001
- From: Thomas Gleixner <[email protected]>
- Date: Thu, 7 Nov 2013 12:21:11 +0100
- Subject: [PATCH] timers: do not raise softirq unconditionally
- Mike,
- On Thu, 7 Nov 2013, Mike Galbraith wrote:
- > On Thu, 2013-11-07 at 04:26 +0100, Mike Galbraith wrote:
- > > On Wed, 2013-11-06 at 18:49 +0100, Thomas Gleixner wrote:
- >
- > > > I bet you are trying to work around some of the side effects of the
- > > > occasional tick which is still necessary despite of full nohz, right?
- > >
- > > Nope, I wanted to check out cost of nohz_full for rt, and found that it
- > > doesn't work at all instead, looked, and found that the sole running
- > > task has just awakened ksoftirqd when it wants to shut the tick down, so
- > > that shutdown never happens.
- >
- > Like so in virgin 3.10-rt. Box is x3550 M3 booted nowatchdog
- > rcu_nocbs=1-3 nohz_full=1-3, and CPUs1-3 are completely isolated via
- > cpusets as well.
- well, that very same problem is in mainline if you add "threadirqs" to
- the command line. But we can be smart about this. The untested patch
- below should address that issue. If that works on mainline we can
- adapt it for RT (needs a trylock(&base->lock) there).
- Though it's not a full solution. It needs some thought versus the
- softirq code of timers. Assume we have only one timer queued 1000
- ticks into the future. So this change will cause the timer softirq not
- to be called until that timer expires and then the timer softirq is
- going to do 1000 loops until it catches up with jiffies. That's
- anything but pretty ...
- What worries me more is this one:
- pert-5229 [003] d..h1.. 684.482618: softirq_raise: vec=9 [action=RCU]
- The CPU has no callbacks as you shoved them over to cpu 0, so why is
- the RCU softirq raised?
- Thanks,
- tglx
- ------------------
- Message-id: <[email protected]>
- |CONFIG_NO_HZ_FULL + CONFIG_PREEMPT_RT_FULL = nogo
- Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
- ---
- include/linux/hrtimer.h | 3 +--
- kernel/hrtimer.c | 31 +++++++------------------------
- kernel/timer.c | 28 +++++++++++++++++++++++++---
- 3 files changed, 33 insertions(+), 29 deletions(-)
- --- a/include/linux/hrtimer.h
- +++ b/include/linux/hrtimer.h
- @@ -461,9 +461,8 @@ extern int schedule_hrtimeout_range_cloc
- unsigned long delta, const enum hrtimer_mode mode, int clock);
- extern int schedule_hrtimeout(ktime_t *expires, const enum hrtimer_mode mode);
- -/* Soft interrupt function to run the hrtimer queues: */
- +/* Called from the periodic timer tick */
- extern void hrtimer_run_queues(void);
- -extern void hrtimer_run_pending(void);
- /* Bootup initialization: */
- extern void __init hrtimers_init(void);
- --- a/kernel/hrtimer.c
- +++ b/kernel/hrtimer.c
- @@ -1695,30 +1695,6 @@ static void run_hrtimer_softirq(struct s
- }
- /*
- - * Called from timer softirq every jiffy, expire hrtimers:
- - *
- - * For HRT its the fall back code to run the softirq in the timer
- - * softirq context in case the hrtimer initialization failed or has
- - * not been done yet.
- - */
- -void hrtimer_run_pending(void)
- -{
- - if (hrtimer_hres_active())
- - return;
- -
- - /*
- - * This _is_ ugly: We have to check in the softirq context,
- - * whether we can switch to highres and / or nohz mode. The
- - * clocksource switch happens in the timer interrupt with
- - * xtime_lock held. Notification from there only sets the
- - * check bit in the tick_oneshot code, otherwise we might
- - * deadlock vs. xtime_lock.
- - */
- - if (tick_check_oneshot_change(!hrtimer_is_hres_enabled()))
- - hrtimer_switch_to_hres();
- -}
- -
- -/*
- * Called from hardirq context every jiffy
- */
- void hrtimer_run_queues(void)
- @@ -1731,6 +1707,13 @@ void hrtimer_run_queues(void)
- if (hrtimer_hres_active())
- return;
- + /*
- + * Check whether we can switch to highres mode.
- + */
- + if (tick_check_oneshot_change(!hrtimer_is_hres_enabled())
- + && hrtimer_switch_to_hres())
- + return;
- +
- for (index = 0; index < HRTIMER_MAX_CLOCK_BASES; index++) {
- base = &cpu_base->clock_base[index];
- if (!timerqueue_getnext(&base->active))
- --- a/kernel/timer.c
- +++ b/kernel/timer.c
- @@ -1443,8 +1443,6 @@
- irq_work_run();
- #endif
- - hrtimer_run_pending();
- -
- if (time_after_eq(jiffies, base->timer_jiffies))
- __run_timers(base);
- }
- @@ -1454,8 +1452,39 @@
- */
- void run_local_timers(void)
- {
- + struct tvec_base *base = __this_cpu_read(tvec_bases);
- +
- hrtimer_run_queues();
- - raise_softirq(TIMER_SOFTIRQ);
- + /*
- + * We can access this lockless as we are in the timer
- + * interrupt. If there are no timers queued, nothing to do in
- + * the timer softirq.
- + */
- +#ifdef CONFIG_PREEMPT_RT_FULL
- + /* On RT, irq work runs from softirq */
- + if (irq_work_needs_cpu()) {
- + raise_softirq(TIMER_SOFTIRQ);
- + return;
- + }
- +
- + if (!spin_do_trylock(&base->lock)) {
- + raise_softirq(TIMER_SOFTIRQ);
- + return;
- + }
- +#endif
- +
- + if (!base->active_timers)
- + goto out;
- +
- + /* Check whether the next pending timer has expired */
- + if (time_before_eq(base->next_timer, jiffies))
- + raise_softirq(TIMER_SOFTIRQ);
- +out:
- +#ifdef CONFIG_PREEMPT_RT_FULL
- + rt_spin_unlock_after_trylock_in_irq(&base->lock);
- +#endif
- + /* The ; ensures that gcc won't complain in the !RT case */
- + ;
- }
- #ifdef __ARCH_WANT_SYS_ALARM
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